I think Sony might filter media that it can't play, so start with something you know if can play like MP3 or JPEGs. Make sure you select the correct media type first. If you accidentally select the Video section and then navigate to Photos, you won't see anything. I know this seems obvious, but many people have a problem with this.

Is your PS3 wired or wireless? If wireless, make sure Multicast is enabled on your router. Please let me know if any of these suggestions help.